Gordon Brown yesterday raised the spectre of Margaret Thatcher's ejection from No 10 as he pressed Tony Blair to give assurances to senior party colleagues on the date of his departure and the process of transition. The chancellor is determined not to let his chance to succeed to the leadership on his terms slip from his grasp, as it has in the past.

He believes he has received worthless private assurances from the prime minister before and now wants flesh on Mr Blair's promise to give his successor "ample time" to take over and set his distinctive agenda. The prime minister gave that assurance under backbench pressure at a meeting with Labour MPs on Monday.

In an interview with GMTV yesterday Mr Brown repeatedly insisted that Mr Blair had said he would talk to senior colleagues about the transition. No 10 regards this demand for a private pledge to senior party colleagues on when he will stand down, and the process of transition, as unreasonable and unworkable.

Mr Brown said in a carefully worded warning: "Tony has said he is going to do it in a stable and orderly way. That means he is going to be talking not just to me, but to senior colleagues about it. Remember when Mrs Thatcher left, it was unstable, it was disorderly and it was undignified."

In the interview, his fourth since Friday, he also said: "There are problems that have got to be sorted out and they have got to be sorted out quickly." His remarks were seen in Downing Street as a coded warning that Mr Blair could yet be thrown out like Lady Thatcher unless he agreed to make a firm private commitment to stand down on a specific date.

Asked if Mr Blair had given him a firm date for the handover, Mr Brown replied: "No, and I think that what he is going to do is to talk to senior colleagues about it."

The chancellor's intervention comes as the latest poll shows Tony Blair to be the most unpopular Labour prime minister in modern times. Only 26% of voters are satisfied with Mr Blair's performance - lower than Harold Wilson's 27% in May 1968 after the pound's devaluation.

According to the YouGov poll for today's Daily Telegraph, Labour now trails the Tories by 6%. The poll puts the Conservatives on 37%, Labour on 31% and the Liberal Democrats on 17%

The Treasury insisted that yesterday's reference by Brown to Lady Thatcher was not a threat, but just a reminder to the party of the dangers of a disorderly transition. Many MPs thought Mr Blair had given ground on Monday by promising that he will give Mr Brown ample time to succeed him. In discussions in recent weeks he has made similar promises to Mr Brown privately, as well as vowing not to support anyone against Mr Brown for the Labour leadership.

Mr Blair's closest aides were admitting yesterday that he would probably now have to go in the summer of 2007 or early 2008 at the latest. But these semi-public assurances are treated with suspicion by the Brownites. They want the assurances on the handover to be passed to some senior cabinet members, and possibly officials or senior members of Labour's national executive. The chancellor is not seeking a public timetable but one to which Mr Blair can be held to account.

An increasingly emboldened chancellor also mapped out the issues he wants to address most urgently in the face of Labour's slumping poll rating. "We are about propriety, we are about security and law and order, and we are about giving people the best public services," he said.

Mr Brown is determined to reach an agreement on what issues he will be responsible for and take the initiative on, and the issues that Mr Blair should be allowed to complete. He also wants assurances that he will be fully consulted on appointments and policy that might impact on his leadership. He feels last week's reshuffle was provocative by promoting Blairites, especially Hazel Blears as party chair.

There is another reason for Mr Brown's anger. His team has been exercised by letters from Mr Blair to cabinet ministers. The letters - being released by No 10 this week and next - are "designed to identify the key challenges for departments and how they propose to deliver against these". But the letter that most concerned the Brownites was one dispatched on Friday - amid the cabinet reshuffle - from Mr Blair to Ms Blears. In what Mr Brown's supporters regard as provocative language, it set out how the party must be organised by 2009-10. Some Brownites now want it withdrawn, saying drily: "It may not be a sensible idea to send a personal mandate to the party chairman to do what whatever she wants."

Mr Brown's team believe the organisation of the party machine must be his preserve because he will take the party into the next election. The letter was discussed in No 10 - as if some were aware that its existence, as much as its contents, might not go down well in No 11.

The letter says: "We need to adopt new ways of working to connect the voters, members and potential supporters. As the opposition parties look to renew themselves, we need to meet this challenge with the same relish and enthusiasm which we created New Labour more than a decade ago."

It calls for a radical review of the way the party is organised and sets out plans to develop a Labour supporters' network, seen by some as a way of circumventing the traditional party member. The letter argues that 100,000 "supporters" have so far been recruited, adding that specialist networks in education and health have been developed which have helped on policy discussion about the education bill. Ms Blears is instructed to produce "imaginative proposals for developing a supporters' network".

The letter argues that the internet rather than the traditional meeting will be the face and the front door of the party to an increasing number of people, particularly the under-35s. It then discusses plans for a "Let's Talk" debate on the future of public service reform, in essence a rebranding of the successful Big Conversation events pioneered by Labour before the last general election. It adds that Mr Blair will launch the debate on May 15, including a pamphlet called Building a Progressive Future. Some of the ideas have been circulated before, but the Brownites regard the letter as a symbol of Mr Blair's reluctance to consult.

Mr Brown was displeased earlier this year when Mr Blair resurrected the issue of Lords reform and state party funding in the wake of the peerages for loans scandal. The Blairites hit back that they had tried to launch a joint consultation with Mr Brown on these issues in January, but were blocked by him on the basis that the ground had not been fully prepared.

Tensions are also bubbling about spending plans. Mr Blair this week again highlighted his plan for a fundamental spending review to be published this summer. It is being drawn up the Treasury, the Cabinet Office and No 10. This would look at all aspects of government spending from first principles on the assumption that no spending is sacrosanct. Mr Blair has always been frustrated that the biennial spending reviews run by the Treasury become the vehicle by which the chancellor dominates domestic policy. The fundamental review is seen by some as Mr Blair's means of trying to get a slice of the spending action.

One other issue has been corroding relations: pensions. On Thursday, the day of the local elections, Mr Brown and Mr Blair had yet another difficult discussion on whether the proposals in the Turner commission's report were affordable. Mr Brown believes the Turner proposal to link the basic state pension to earnings is unaffordable. The white paper is due by the end of the month.

If the week's events end with Mr Brown and Mr Blair agreeing to cooperate once more, they will have to look at why previous talks on the transition failed, leaving an atmosphere of mistrust. Discussions were held in January and February, with Alastair Campbell and Philip Gould representing Mr Blair, and Ed Miliband and Ed Balls representing Mr Brown. According to one account, the talks were relatively formal, with 15 or 16 minuted meetings. The committee - dubbed the capitulation committee by No 10 - also looked at the issues Mr Brown should gradually take up, including security, the environment and globalisation. Accounts differ as to why the meetings ended, but some Brownites found it strange that they were not talking to people inside No 10, rather than two admittedly hugely influential figures who flitted in and out of No 10.

No one is planning to restart such a process, or at least not with the same personnel. This is partly because the Brownites feel they have the upper hand as the mood on the backbenches shifts away from Mr Blair, spurred by disenchantment with the education bill and the manner of its introduction.

It is significant that the backbench figures who have tipped the balance away from Mr Blair this week have been key figures in the alternative education white paper - Nick Raynsford, John Denham, Alan Whitehead and Richard Burden. It is their disillusionment with Mr Blair's brand of public sector reform that has weakened him. The new education secretary, Alan Johnson, has less than three weeks to find a deal with the rebels before the bill returns to the Commons for its third reading.

As one backbencher warned: "If we don't get what we want on the transition, we can make this place ungovernable.

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